Charge particle beam microscopy, such as scanning ion microscopy and electron microscopy, provides significantly higher resolution and greater depth of focus than optical microscopy. In a scanning electron microscope (SEM), a primary electron beam is focused to a fine spot that scans the surface to be observed. Secondary electrons are emitted from the surface as it is impacted by the primary electron beam. The secondary electrons are detected, and an image is formed, with the brightness at each point on the image being determined by the number of secondary electrons detected when the beam impacts a corresponding spot on the surface. Scanning ion microscopy (SIM) is similar to scanning electron microscopy, but an ion beam is used to scan the surface and eject the secondary electrons.
In a transmission electron microscope (TEM), a broad electron beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site. Samples are typically less than 100 nm thick.
In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the work piece are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface.
Because a sample must be very thin for viewing with transmission electron microscopy (whether TEM or STEM), preparation of the sample can be delicate, time consuming work. The term “TEM” sample as used herein refers to a sample for either a TEM or an STEM and references to preparing a sample for a TEM are to be understood to also include preparing a sample for viewing on an STEM. One method of preparing a TEM sample is to cut the sample from a substrate using an ion beam. A probe is attached to the sample, either before or after the sample has been entirely freed. The probe can be attached, for example, by static electricity, FIB deposition, or an adhesive. The sample, attached to the probe, is moved away from the substrate from which it was extracted and typically attached to a TEM grid using FIB deposition, static electricity, or an adhesive.
FIG. 1 shows a typical TEM grid 100, which comprises a partly circular 3 mm ring. In some applications, a sample 104 is attached to a finger 106 of the TEM grid by ion beam deposition or an adhesive. The sample extends from the finger 106 so that in a TEM (not shown) an electron beam will have a free path through the sample 104 to a detector under the sample. The TEM grid is typically mounted horizontally onto a sample holder in the TEM with the plane of the TEM grid perpendicular to the electron beam, and the sample is observed.
Some dual beam systems include an ion beam that can be used for extracting the sample, and an electron beam that can be used for SEM or STEM observation. In some dual beam systems, the FIB is oriented an angle, such as 52 degrees, from the vertical and an electron beam column is oriented vertically. In other systems, the electron beam column is tilted and the FIB is oriented vertically or also tilted. The stage on which the sample is mounted can typically be tilted, in some systems up to about 60 degrees.
TEM samples can be broadly classified as “planar view” samples or “cross sectional view” samples, depending on how the sample was oriented on the work piece. If the face of the sample to be observed was parallel to the surface of the work piece, the sample is referred to as a “planar view” sample. If the face to be observed was perpendicular to the work piece surface, the sample is referred to as a “cross sectional view” sample.
FIG. 2 shows a cross-sectional view TEM sample 200 that is partly extracted from a work piece 202 using a typical process. An ion beam 204 cuts trenches 206 and 208 on both side of sample to be extracted, leaving a thin lamella 210 having a major surface 212 that will be observed by an electron beam. The sample 200 is then freed by tilting the work piece 202 in relation to an ion beam, and cutting around its sides and bottom. A probe 216 attaches to the top of the sample 200, before or after it is freed, and transports the sample to a TEM grid. FIG. 2 shows sample 200 almost entirely freed, remaining attached by a tab 218 on one side. FIG. 2 shows ion beam 204 ready to sever tab 218.
As shown in FIG. 2, the major surface 212 is oriented vertically. Transporting the lamella typically does not change its orientation, so its major surfaces are still oriented vertically when the sample 200 is brought to a TEM sample holder. The plane of the TEM grid 100 is typically oriented vertically as shown in FIG. 3, so that the sample 200 can be attached to the TEM grid in such a way that major surface 212 extends parallel to the plane of the grid, and the grid structure will not interfere with the transmission of electrons when the grid is mounted in a TEM. The ion beam can be used to attach the extracted sample to the TEM grid by ion beam deposition. Once attached, the face of the sample 200 can also be thinned using the ion beam. FIG. 3 shows the sample 200 being attached to the TEM grid 100 in a grid support 302 on a sample stage 304. Sample 200 is attached to grid using an ion beam 204 and a deposition precursor gas 310 from a nozzle 312. FIG. 4 shows that the stage 304 is rotated and tilted so that the sample 200 is perpendicular to the ion beam 204 so that the sample 200 can be thinned by the ion beam.
FIG. 5 shows a work piece 500 from which a planar view sample 502 is being extracted to view a face 504 of the sample. The sample 502 is undercut by two intersecting ion beam cuts 506A and 506B from opposite directions, and then the ion beam cuts the sides 508A and 508B to free a “chunk.” A probe 510 is attached to the top of the sample 502. The extracted sample is therefore oriented horizontally. If the sample were attached in a horizontal orientation to a vertically oriented TEM grid, the sample would extend normal to the plane of the grid, and the grid would interfere with the electron beam of the TEM. If the sample were mounted in a horizontally oriented TEM grid, the face 504 to be observed would face upward. It would then be difficult in a conventional FIB system to thin the back side of the planar sample 502 without removing the TEM grid from the vacuum chamber and flipping it over to expose the back side of sample 502 for thinning.
This problem of the orientation of a planar view TEM sample 502 has been overcome in the past by using a “flip stage,” on which the TEM grid can be oriented horizontally for attaching the planar view sample, and then the stage can be flipped 180 degrees and rotated so that the backside of the sample can be presented normal to the ion beam for thinning. A flip stage is described for example in U.S. Pat. App. Pub. No. 20040144924 of Asselbergs et al. for “Method for the manufacture and transmissive irradiation of a sample, and particle-optical system” and provides a degree of freedom not available on conventional stages. Such flip stages are not available in all FIB systems.
Thus, it is desirable to provide a method and apparatus for attaching a planar view sample to a TEM grid in a manner such that the sample can be thinned without reorienting the TEM sample holder.